The present invention relates to analyzing the operation of an internal combustion engine. In particular, the invention relates to a device for sensing the temperature in the cylinders of an internal combustion engine and to a method of adjusting the air-fuel ratio depending upon the temperature sensed.
The internal combustion engine belongs to the general class of engines known as heat engines. Heat engines are operated by heat, some of which is converted to useful work. In internal combustion engines the heat is produced by burning an air-fuel mixture in the cylinders. The air-fuel mixture is usually ignited by a spark from a spark plug mounted in the cylinder head.
The temperature in the cylinder of an internal combustion engine is an indicator of certain engine parameters. Specifically, it is well known that the temperature of the combusting materials in the cylinders of an internal combustion engine is an indication of the stoichiometry of the chemical reaction taking place. As an air-fuel mixture approaches its stoichiometrically correct composition, more heat is generated per cycle, and also less fuel is discharged from the cylinder unburned. Thus, the temperature in a cylinder of an internal combustion engine is an indicator of the engine efficiency and the air-fuel ratio of the mixture supplied to the cylinder.
Several ways to measure the temperature in the cylinder of an internal combustion engine are known. Certain of these methods require modification of the engine. A method of measuring the temperature in an internal combustion engine without modifying the engine is shown in U.S. Pat. No. 3,754,139. This patent discloses a device capable of measuring the infra-red radiation given off by the air-fuel mixture combusting in the cylinder. The infra-red detector is placed over a spark plug having a transparent insulator which allows the infra-red radiation to pass from the burning air-fuel mixture to the infra-red detector. The amount of infra-red radiation given off can be used to determine approximate flame temperature. However, the infra-red detector cannot discriminate among sources of infra-red radiation it receives. There are various sources of infra-red radiation in an internal combustion engine such as the burning air-fuel mixture, the exhaust valve, and the cylinder walls. In addition, these sources of infra-red radiation can have different and changing emissivity coefficients due to a buildup of corrosion or a carbon coating. These changing factors can affect the reliability of the infra-red detector.
U.S. Pat. No. 935,154 discloses the use of a thermocouple to measure the temperature in the cylinder of an internal combustion engine. The thermocouple is mounted in a spark plug specially designed to accommodate the thermocouple. The spark plug has a ground electrode extending from the threaded base of the spark plug as is typical in conventional spark plugs. However, the second electrode is offset from the axis of the plug in a direction perpendicular to its axis. A thermocouple extends through the insulator bushing adjacent to the offset electrode.
Most engine manufacturers recommend a spark plug for use in the engine. The use of a non-recommended plug will affect engine performance. The geometry of the spark plug described in U.S. Pat. No. 935,154 is not the geometry of a spark plug which is recommended for any engine. More specifically, a recommended spark plug has an axially located center electrode and electrode geometry specifically designed for the application for which it is intended. The configuration of the electrode in a recommended spark plug determines many operating characteristics of a spark plug which are important for normal engine operation. The use of a spark plug in an engine which is not the spark plug recommended for the engine will result in abnormal engine operation.
Another method of determining whether an engine is receiving the proper air-fuel ratio may be considered a manual method. Some experienced individuals can look at a spark plug and determine whether the proper air-fuel ratio is being delivered to the engine by the color of the deposit on the porcelain of the spark plug. This, of course, is subject to human error and requires substantial experience.